1. Field of the Invention
This invention relates generally to data processing, and more particularly, to decryption of scrambled data stored in prescribed regions of a mass storage medium, such as a CD-ROM.
2. Description of Related Art
All computers, such as the various models of personal computers, or PC's, produced by IBM, corporation execute "operating system software" that instructs the PC on how to use other programs, termed "application software," such as word processing and spreadsheet programs. Examples of PC operating systems include MS-DOS and WINDOWS, manufactured by Microsoft Corporation, and IBM's OS/2.
Before a PC can run an operating system, it must load the operating system from a disk to the PC's working memory which is ordinarily random access semiconductor memory (RAM). This is carried out through a process known as "bootstrapping," or more simply, "booting" the PC. Booting occurs automatically when the PC is first turned on, a process called a "cold boot," or by the user while the computer is running ("warm boot").
Bootstrapping performs only two functions, the first being to run a power-on self-test, or POST, and the other to search the storage media (floppy diskette or hard disk) for the operating system it will load. These functions are controlled by firmware stored in one or more basic input-output system, or BIOS, chips inside the PC.
The POST initializes all the internal hardware and hardware connected to the PC known to it, such as memory, and tests and places the hardware in an operational state. The BIOS program then normally checks drive A of the PC to determine if it contains a formatted floppy disk. If a disk is mounted in the drive, the program searches specific locations on the disk to determine if a valid boot record exists. If the floppy drive is empty, the boot program checks the hard drive C for the system files. In the absence of these files, the BIOS will generate an error message.
After locating a disk with a valid boot record, the BIOS program reads the data stored on the first sector of the disk, and copies that data to specific locations in RAM. This information, found in the same location on every formatted disk, constitutes the DOS boot record. The BIOS then passes control to the boot record which instructs the PC on how to load the two hidden operating system files to RAM (the files named IBMBIO.COM and IBMDOS.COM on IBM computers). After loading other operating system files into RAM to carry out the rest of the boot up sequence, the boot record is no longer needed.
The root directory of the boot disk is next searched for a file created by the user (CONFIG.SYS) and which contains commands instructing the operating system how to handle certain operations such as how many files may be opened at a time, and so-called device drivers which describe the existence and characteristics of hardware devices not recognized by BIOS that may be connected to the PC.
Next loaded from the boot disk into RAM is the file COMMAND.COM which is an operating system file containing, among other functions, fundamental DOS commands used throughout application program execution, and a file named AUTOEXEC.BAT created by the user and containing a series of DOS batch file commands or program names to be executed by the PC each time the computer is turned on. This completes the boot up sequence, and at this point the computer is considered fully booted and ready to be used.
The usual types of storage media, in order of storage density are floppy disk, hard disk storage, which are magnetic media, and more recently, CD-ROM which is an optical medium capable of storing a considerable amount of data. A CD-ROM is a "read only" medium, although magneto-optical media, capable of being written to as well as read from, are emerging. Presently, floppy disks are 5.25 or 3.5 inches in diameter, with smaller sizes now emerging, and are capable of storing up to 2.88 megabytes of data. Hard disks store considerably more data, currently in the range of up to 2 gigabits, arranged as clusters disbursed throughout the medium.
CD-ROM's have attributes of both hard disks and floppy diskettes, that is, they have the storage capacity of a hard drive and the replaceability of a diskette. CD-ROM technology currently is capable of storing more than 600 megabytes of read-only data along a continuous spiral track on a phonograph record-like optical medium. As with other disk media, the CD divides its capacity into short segments, or "large frames" for addressing. The number of such large frames varies but can reach about 315,000, each containing 2352 bytes under a defined media standard. Most of the frame contains data and the remainder is divided among a synchronization field, sector address tag field and an auxiliary field. Data stored on the CD-ROM is formatted as sectors of data comprising 800 h bytes each, and with boot record residing at a prescribed (11 h) sector in the last session on the CD. The boot record points to a boot catalog providing descriptions and locations of disk or diskette image emulations recorded on the CD-ROM, one or more of which may be bootable and may carry an operating system.
Image formats of these three types of media are hierarchical in structure, with diskette lowest and CD-ROM highest, and each medium adding regions of storage overlying the storage regions of the medium beneath it. Referring to FIG. 1, which depicts the image formats of the three media, a floppy diskette starts with a boot record, which is a short program loading the operating system into the main memory, followed by a pair of file allocation tables (FAT's) which record the file structure of the diskette (two FAT's are often provided, as shown, to perform an integrity check on stored files). Next is a root directory which records the files stored on the diskette, and finally the data storage region.
The structure of a fixed disk includes not only the same regions as found in a diskette, but precedes with a reserved region as well as a partition table which, as the first record on the disk, defines the operating boundaries constituting the logical partitions of the disk, if the disk is partitioned by the user. The hard file may contain multiple "floppy images."
A CD-ROM adds to that a table of descriptors, such as defined by ISO 9660, as the first record on the CD. How data is organized into files stored on a CD-ROM is designated by the ISO 9660 specification, which specifies, among other things, that a "Primary Volume Descriptor" must reside at sector 10 h relative to the start of a session, followed by any number of other Volume Descriptors, and then by a "Volume Descriptor Terminator." The boot catalog, following the ISO descriptors, is a directory pointing to the stored images as files as identifying image characteristics. Incorporation by reference is made herein to the ISO 9660 specification in its entirety. A CD may contain multiple "hard file" and/or "floppy images. "
The ISO 9660 specification provides, optionally, new boot capabilities for personal computers, a concept disclosed in the "El Torito" bootable CD-ROM format specification, Version 1.0, recently jointly developed and made publically available by Phoenix Technologies and IBM Corporation, the assignee of the present invention. This specification, incorporated herein by reference, describes how the BIOS boot procedure can be enhanced to support the CD-ROM using INT 13 calling conventions for enabling the CD-ROM to boot as the A drive or C drive without device drivers. In accordance with this specification, if the user selects one of the boot CD-ROM options on a setup menu, and during POST the BIOS detects the presence of a CD-ROM drive, INT 19 at the end of POST will attempt to load the operating system using the boot sequence specified in Setup. To accomplish this, the installable boot CD-ROM feature makes available one of two INT 19 functions, namely, single image and multiple image INT 19.
The single image INT 19 feature accesses the booting catalog in the CD-ROM header, verifies the existence of a boot image on the CD-ROM and reads the initial/default entry and then boots from the disk image specified in this entry. Multiple image INT 19, if instructed to boot from the CD-ROM, accesses the booting catalog, verifies, and then boots from either the image specified in the initial default entry or from one of the other images listed in the section headers and section entries that follow the initial/default entry.
Three types of CD-ROM configuration are shown in FIGS. 2(a)-2(c). In FIG. 2(a), the normal CD-ROM configuration is not bootable; it uses root directory and CD-ROM drivers to access CD-ROM images. In FIG. 2(b), a BIOS with a single boot image capability accesses the initial/default entry to access a single bootable disk image. After loading the operating system, the system can revert to standard CD-ROM drivers and the root directory to access CD-ROM images. In FIG. 3, a BIOS with multiple boot-image capability can access any one of a number of bootable disk images listed in the booting catalog. After loading the operating system, the system can access other items in the disk image with standard INT 13 calls or return to normal access of CD-ROM images using CD-ROM drivers and the root directory.
The operating environment is depicted in FIG. 5, wherein the right hand side is a map of the first 1M of random access main memory. The lowest address of the main memory is occupied by BIOS data, such as interrupt vectors and other machine specific operating parameters such as identification of drives and access to them through BIOS INT 13 functions. The next region of memory is occupied by the operating system, such as DOS, followed by an extended BIOS area, or BIOS EBDA entered during booting. Residing in the upper layers of the main memory are video data and firmware based BIOS which is fixed for all applications. The usual BIOS functions do not recognize a CD-ROM drive, which is instead defined by ISO-9660 device driver software residing in the operating system, as shown in FIG. 5.
In FIG. 6, in accordance with the El Torito specification, enhancements made to BIOS EBDA enable the INT 13 calls to recognize the CD-ROM and further to provide for hard disk and floppy diskette image emulations derived from the CD-ROM storage files, that is, to treat the emulation images as if they are physical drives. In this example, the CD-ROM image which is booted becomes the A (floppy diskette) drive, and the physical A drive is renamed as drive B (A: becomes B; floppy 1. IMB becomes A; DOS and application load from emulated A).
An important advantage of bootable CD-ROM's in accordance with the El Torito specification is the capability of distributing multiple applications on a single medium. Multiple versions of the same application in different languages can also be distributed; the user simply boots the CD-ROM and then selects the operation system, application set, and/or language to install.
However, one problem with this approach is in management of royalties or other distribution control. For example, several environments, such as IBM DOS, MS DOS, Windows, OS/2, AIX and others can be on the same CD-ROM but the user may not have purchased all of them. Some form of protection accordingly is needed to prevent unauthorized access to some parts of the CD-ROM.